Hydro fluorocarbons (HFS) have been identified as commercially-viable substitutes for chlorofluorocarbons in various applications. For example, difluoromethane (HFC-32) is useful as a refrigerant, blowing agent, cleaning agent, and aerosol propellant. Such HFS are prepared commonly by fluorinating chlorinated organic starting materials, such as dichloromethane, using a fluorination agent, such as hydrogen fluoride. Although fluorination has proved to be a convenient method for preparing HFS, particularly HFC-32, the introduction of chlorinated impurities in the product stream is generally unavoidable. Such chlorinated impurities include, for example, starting materials and intermediates from incomplete fluorination and/or by-products from undesirable side reactions. These chlorinated impurities diminish HFC-32's purity.
Although significant purification of HFC-32 can be achieved using conventional distillation, certain chlorinated impurities such as dichlorodifluoromethane (CFC-12) and methyl chloride (HCC-40) tend to form azeotropic or near-azeotropic mixtures with HFC-32 thereby rendering conventional distillation impossible or impractical. Generally, if the relative volatility of the product to the impurities in the mixture 15 is close to 1.0, conventional distillation is not practical. The relative volatility α of an impurity 2 to a fluorinated compound 1 in a mixture is defined herein according to the following equation:
      α          2      ,      1        =                    y        2                    x        2              ×                  x        1                    y        1            where x and y are, respectively, the liquid and vapor mole fraction of the components. Therefore, for a mixture of HFC-32 and a chlorinated impurity, such as CFC-12 and/or HCC-40, where the relative volatility is close to 1.0, a separation technique other than conventional distillation is desirable.
Alternatives to conventional distillation for separating azeotropic or near-azeotropic mixtures include adsorption, membrane diffusion, and extractive distillation. Perhaps the most popular of these approaches is extractive distillation. For example, in Japanese Patent Publication No. 7-291878, an extractive distillation technique is disclosed in which an extractive agent selected from 1,1-dichloro-1-fluoroethane, 2,2-dichloro-1,1,1-fluoroethane, trichlorotrifluoroethane, and dichloropentafluoropropane is used in separate 1,1,1-trifluoroethane, pentafluoroethane or methyl chloride from HFC-32. Unfortunately, despite their effectiveness, the extractive agents described above nevertheless add impurities to the process which must be removed through additional distillation steps. These additional distillation steps add cost and complexity to the process.
Therefore, a need exists for a process of purifying HFC-32 without adding extractive agents that complicate the process and necessitate additional distillation steps. The present invention fulfills this need among others.